The present invention relates to an apparatus intended for purifying paper pulp by screening it in the liquid state.
There are at present a very large number of apparatus assuming this function. They generally comprise:
a closed tank,
a cylindrical screen inside this tank, the screen being pierced either by round holes or by elongated fine slits,
a rotor having profiled blades which move in the vicinity of and along the surface of the screen at a suitable linear speed,
nozzles for the admission of the crude pulp (unpurified), the evacuation of the purified pulp and the rejects stopped by the screen.
It may function either by centrifugal or by centripetal forces, depending on whether the pulp passes from the interior toward the exterior of the screen or in the reverse path.
Apparatus of this type are described in French Patent Nos. 1,271,054 and 1,546,515.
As the moving blades move in the liquid, they create around them zones of super pressure and reduced pressure which unclog the screen continuously owing to a to and fro movement of the liquid in its vicinity.
Without the continuous action of the rotor blades, the screen blocks immediately due to the accumulation of fibres and dirt on its surface. The unclogging blades are generally situated on the upstream face (in the direction of flow of the liquid) of the screen, that is to say, inside it for centrifugal operation and outside it for centripetal operation.
Centripetal apparatus in which the blades are situated on the downstream face of the screen, that is to say inside it, are however known. This last arrangement is particularly suitable when the openings in the screen are slits, and in this case it is already known that operation is improved by using slits having an asymmetrical clearing.
The dimensions of the perforations most generally used are from 1.5 to 2.5 mm in diameter for holes and from 0.25 to 0.7 mm in width for slits.
The concentration of the paper pulp (ratio of the weight of the dry fibres to the weight of water in the mixture) at which the known apparatus of this type utilising perforations of these sizes, function is generally low, of the order of 1%, and rarely exceeds 1.5 to 2%. The weakness of these concentrations is often inconvenient. In fact, in a scheme for the treatment of paper pulp, especially pulps obtained from old paper, it is of considerable interest to purify the pulp before refining it, that is to say to stop the contaminants before dividing them by refining. Now the minimum concentration for refining in a suitable manner is from 3 to 4%, and in density this demands the pulp after it has passed through purifiers, this operation often being expensive and cumbersome.